It is estimated that 15 to 25 million people worldwide are infected with human T-cell leukemia virus type 1 (HTLV-1) or type 2 (HTLV-2). HTLV-1 infection is associated primarily with leukemia and neurological disease in a small percentage of infected patients, whereas HTLV-2 disease association is less clear. Although infected patients develop antibody and cytotoxic T-lymphocyte (CTL) responses to many of the viral proteins, HTLV manages to persist throughout life. In the previous funding period we generated and characterized distinct Tax-2 mutants that failed to activate NFkB/Rel or CREB/ATF cellular activation pathways and developed a HTLV-2 infectious virus that replicates independently of Tax. These reagents provided us with the unique tools to directly implicate Tax in the transformation of primary T-cells. Furthermore, we showed that Tax deregulation of NFkB/Rel was absolutely critical for cellular transformation, and that Tax activation of CREB/ATF also contributed, but at a later step in the process. We also observed that the in vitro transformation tropism of HTLV-1 and HTLV-2 is preferential for CD4+ and CD8+ T-cells, respectively, and the first successful recombinant virus studies between HTLV-1 and HTLV-2 indicated that this distinct transformation tropism is not encoded by Tax or Rex. Our long-range goal is to understand the molecular basis of T-lymphocyte transformation and induction of leukemia/lymphoma by HTLV. We continue to feel that the best way to ultimately understand the pathogenesis of HTLV is to use infectious molecular clones of HTLV-1 and the related less pathogenic HTLV-2, manipulate these clones to contain specific alterations or gene mutations, and compare and contrast their biological properties in primary human T-cells in vitro and our reproducible rabbit model in vivo. In Aim 1 of this proposal we will test the hypothesis that additional documented activities of Tax that include the induction of micronuclei/DNA damage and direct interaction with tumor suppressor and cell cycle regulatory proteins are critical for viral replication and HTLV-mediated cellular transformation and IL-2 independent growth. In Aim 2 we will extend our novel HTLV-1 and HTLV-2 tropism studies to determine the viral genetic basis for the distinct transformation tropisms of HTLV-1 (preference for CD4+ T-cells) and HTLV-2 (preference for CD8+ Tcells). Lastly, the rabbit model will be used to assess early infectivity tropism in a newly infected host and evaluate key Tax virus mutants and HTLV-1 and HTLV-2 recombinants for tropism, viral replication kinetics, immune response, and viral persistence. These studies will provide important new information about mechanisms by which HTLV Tax mediates cellular transformation and provide a greater understanding of the distinct biological properties of HTLV-1 and HTLV-2.